1. Field of the Invention
The present invention relates generally to an orthogonal frequency division multiplexing (OFDM) communication system, and in particular, to an apparatus and method for generating a preamble sequence using a Golay complementary sequence.
2. Description of the Related Art
In general, a wireless communication system supporting a wireless communication service is comprised of Node Bs and user equipments (UEs). The Node Bs and the UEs support a wireless communication service using transmission frames. Therefore, the Node Bs and the UEs must acquire mutual synchronization for transmission and reception of transmission frames, and for the synchronization acquisition, a Node B transmits a synchronization signal so that a UE can detect the start of a frame transmitted by the Node B. The UE then detects frame timing of the Node B by receiving the synchronization signal transmitted by the Node B, and demodulates the received frames according to the detected frame timing. Commonly, a specific preamble sequence previously appointed by the Node B and the UE is used for the synchronization signal.
In addition, a preamble sequence having a low peak-to-average power ratio (PAPR) is used as the preamble sequence in an OFDM communication system, and a preamble created by concatenating a long preamble, which is necessary for performing coarse synchronization, to a short preamble, which is necessary for performing fine frequency synchronization, is used as the preamble transmitted from a Node B to a UE. Further, only the short preamble is used for the preamble transmitted from the UE to the Node B to acquire fine frequency synchronization. The reason the preamble sequence having a low PAPR must be used as a preamble sequence of the OFDM communication system will be described herein below.
Because the OFDM communication system, which is a multicarrier communication system, uses a plurality of carriers, i.e., a plurality of subcarriers, orthogonality between the subcarriers is important. Therefore, phases of the subcarriers are appropriately set so that orthogonality there between is secured, and if the phases are changed during signal transmission/reception through the subcarriers, signals on the subcarriers overlap each other. In this case, the amplitude of the signals that overlap due to the phase change deviates from a linear region of an amplifier included in the OFDM communication system, disabling normal signal transmission/reception. This is the reason why the OFDM communication system uses a preamble sequence having a minimal PAPR.
Further, the OFDM communication system transmits data for several users, or UEs, by time-multiplexing one frame. In the OFDM communication system, a frame preamble indicating a start of a frame is transmitted for a predetermined period beginning at a start point of the frame. Because data may be irregularly transmitted to the respective UEs within one frame, a burst preamble indicting the start of data is located at a front part of each data block. Therefore, a UE must receive a data frame in order to identify a transmission start point of the data. The UE should be synchronized to a start point of data in order to receive the data, and to this end, the UE must acquire a preamble sequence used in common by all systems for synchronization before receiving signals.
The OFDM communication system is identical to a non-OFDM communication system in a source coding scheme, a channel coding scheme, and a modulation scheme. While a code division multiple access (CDMA) communication system spreads data before transmission, the OFDM communication system performs inverse fast Fourier transform (IFFT) on data and inserts a guard interval in the IFFT-transformed data before transmission. Therefore, compared with the CDMA communication system, the OFDM communication system can transmit a wideband signal with relatively simple hardware.
Further, in the OFDM communication system, if a parallel bit/symbol stream generated by parallel converting a plurality of serial bit/symbol streams is applied as a frequency-domain IFFT input after modulation is performed on data, an IFFT-transformed time-domain signal is output. The time-domain output signal is obtained by multiplexing a wideband signal with several narrowband subcarrier signals, and a plurality of modulation symbols are transmitted for a one-OFDM symbol period through the IFFT process.
However, in the OFDM communication system, if the IFFT-transformed OFDM symbol is transmitted as it is, interference between a previous OFDM symbol and a current OFDM symbol is unavoidable. In order to remove the inter-symbol interference, a guard interval is inserted. The guard interval is used to insert null data for a predetermined period. However, in a method of transmitting null data for the guard interval, if a receiver incorrectly estimates a start point of an OFDM symbol, interference occurs between subcarriers, causing an increase in an error probability of a received OFDM symbol. Therefore, a “cyclic prefix” scheme or a “cyclic postfix” scheme has been proposed for the guard interval. In the former scheme, last 1/n bits in a time-domain OFDM symbol are copied and then inserted in an effective OFDM symbol, and in the latter scheme, first 1/n bits in a time-domain OFDM symbol are copied and then inserted in an effective OFDM symbol. A receiver may acquire time/frequency synchronization of a received OFDM symbol using a characteristic of the guard interval created by copying a part of the one time-domain OFDM symbol, i.e., a beginning part or a last part of one OFDM symbol, and then repeatedly arranging the copied OFDM symbols.
In any radio frequency (RF) system, a transmission signal transmitted by a transmitter is distorted while it passes through a radio channel, and thus, a receiver receives a distorted transmission signal. The receiver acquires time/frequency synchronization of the received distorted transmission signal, using a preamble sequence previously set between the transmitter and the receiver, performs channel estimation, and then demodulates the channel-estimated signal into frequency-domain symbols through fast Fourier transform (FFT). After demodulating the channel-estimated signal into frequency-domain symbols, the receiver performs channel decoding and source decoding corresponding to the channel coding applied in the transmitter on the demodulated symbols, to thereby decode the demodulated symbols into information data.
The OFDM communication system uses a preamble sequence for all frame timing synchronization, frequency synchronization, and channel estimation. The OFDM communication system may perform frame timing synchronization, frequency synchronization, and channel estimation using a guard interval and a pilot subcarrier in addition to the preamble. The preamble sequence is used to transmit known symbols at a beginning part of every frame or data burst, and update estimated time/frequency/channel information at a data transmission part, using information on the guard interval and the pilot subcarrier.
FIG. 1 is a diagram illustrating a structure of a long preamble sequence for a conventional OFDM communication system. It should be noted that a current OFDM communication system uses the same preamble sequence in both a downlink (DL) and an uplink (UP). Referring to FIG. 1, in the long preamble sequence, a length-64 sequence is repeated 4 times and a length-128 sequence is repeated 2 times, and in the light of a characteristic of the OFDM communication system, the above-stated cyclic prefix (CP) is added to a front end of the 4 repeated length-64 sequences and to a front end of the 2 repeated length-128 sequences. In addition, as described above, signals obtained before performing IFFT are frequency-domain signals, while signals obtained after performing IFFT are time-domain signals. The long preamble sequence illustrated in FIG. 1 represents a time-domain long preamble sequence obtained after performing IFFT.
Frequency-domain long preamble sequences obtained before IFFT are illustrated below.
S(−100:100) ={+1+j, 0, 0, 0, +1+j, 0, 0, 0, +1+j, 0, 0, 0, +1−j, 0, 0, 0, −1+j, 0, 0, 0, +1+j, 0, 0, 0, +1+j, 0, 0, 0, +1+j, 0, 0, 0, +1−j, 0, 0, 0, −1+j, 0, 0, 0, +1+j, 0, 0, 0, +1+j, 0, 0, 0, +1+j, 0, 0, 0, +1−j, 0, 0, 0, −1+j, 0, 0, 0, +1−j, 0, 0, 0, +1−j, 0, 0, 0, +1−j, 0, 0, 0, −1−j, 0, 0, 0, +1+j, 0, 0, 0, −1+j, 0, 0, 0, −1+j, 0, 0, 0, −1+j, 0, 0, 0, +1+j, 0, 0, 0, −1−j, 0, 0, 0, 0, 0, 0, 0, −1−j, 0, 0, 0, +1−j, 0, 0, 0, +1+j, 0, 0, 0, −1−j, 0, 0, 0, −1+j, 0, 0, 0, +1−j, 0, 0, 0, +1+j, 0, 0, 0, −1+j, 0, 0, 0, +1−j, 0, 0, 0, −1−j, 0, 0, 0, +1+j, 0, 0, 0, −1+j, 0, 0, 0, −1−j, 0, 0, 0, +1+j, 0, 0, 0, +1−j, 0, 0, 0, −1−j, 0, 0, 0, +1−j, 0, 0, 0, +1+j, 0, 0, 0, −1−j, 0, 0, 0, −1+j, 0, 0, 0, −1+j, 0, 0, 0, −1−j, 0, 0, 0, +1−j, 0, 0, 0, −1+j, 0, 0, 0, +1+j}*sqrt(2)*sqrt(2)P(−100:100) ={−1, 0, +1, 0, +1, 0, +1, 0, +1, 0, −1, 0, −1, 0, +1, 0, −1, 0, +1, 0, −1, 0, −1, 0, +1, 0, +1, 0, −1, 0, +1, 0, −1, 0, +1, 0, −1, 0, +1, 0, −1, 0, +1, 0, +1, 0, −1, 0, +1, 0, −1, 0, −1, 0, +1, 0, −1, 0, −1, 0, −1, 0, +1, 0, +1, 0, −1, 0, +1, 0, +1, 0, +1, 0, −1, 0, +1, 0, +1, 0, −1, 0, −1, 0, −1, 0, +1, 0, +1, 0, +1, 0, +1, 0, +1, 0, +1, 0, +1, 0, 0, 0, −1, 0, −1, 0, +1, 0, −1, 0, −1, 0, +1, 0, +1, 0, +1, 0, −1, 0, +1, 0, +1, 0, +1, 0, −1, 0, −1, 0, −1, 0, −1, 0, −1, 0, −1, 0, +1, 0, −1, 0, −1, 0, −1, 0, −1, 0, −1, 0, −1, 0, +1, 0, +1, 0, +1, 0, −1, 0, +1, 0, −1, 0, +1, 0, +1, 0, −1, 0, +1, 0, +1, 0, +1, 0, −1, 0, −1, 0, −1, 0, −1, 0, −1, 0, +1, 0, −1, 0, −1, 0, +1, 0, −1, 0, −1, 0, +1, 0, −1}*sqrt(2)*sqrt(2)
Numerals specified in the frequency-domain long frequency sequences S(−100:100) and P(−100:100) represent the subcarriers' positions applied while IFFT is performed, and a detailed description thereof will be made with reference to FIG. 3. S(−100:100) represents a frequency-domain preamble sequence obtained by repeating a length-64 sequence 4 times, and P(−100:100) represents a frequency-domain preamble sequence obtained by repeating a length-128 sequence 2 times. In the expression of S(−100:100) and P(−100:100), ‘sqrt(2)’ means ‘root 2’, and ‘sqrt(2)*sqrt(2)’ means performing double amplification to increase transmission power of S(−100:100) and P(−100:100).
FIG. 2 is a diagram illustrating a structure of a short preamble sequence for a conventional OFDM communication system. Referring to FIG. 2, in the short preamble sequence, a length-128 sequence is repeated 2 times, and in the light of a characteristic of the OFDM communication system, the above-stated cyclic prefix (CP) is added to a front end of the 2 repeated length-128 sequences. In addition, the short preamble sequence illustrated in FIG. 2 represents a time-domain short preamble sequence obtained after performing IFFT, and a frequency-domain short preamble sequence equals the above-stated P(−100:100).
The long preamble sequence described above must be generated taking the following conditions into consideration.
(1) The long preamble sequence should have a low PAPR.
In order to maximize transmission efficiency of a power amplifier (PA) in a transmitter of an OFDM communication system, a PAPR of an OFDM symbol must be low. That is, because an IFFT-transformed signal is applied to a power amplifier and because of a non-linear characteristic of the power amplifier, a low PAPR is required. A PAPR of an OFDM symbol must be low in a ratio of maximum power to average power of a time-domain OFDM symbol corresponding to an IFFT processor's output terminal of the transmitter, and for a low ratio of the maximum power to the average power, uniform distribution must be provided. In other words, a PAPR of an output becomes low if symbols having a low cross correlation are combined in an IFFT processor's input terminal of the transmitter, i.e., in a frequency domain.
(2) The long preamble sequence should be suitable for parameter estimation needed for communication initialization.
The parameter estimation includes channel estimation, frequency offset estimation, and time offset estimation.
(3) The long preamble sequence should have low complexity and low overhead.
(4) Coarse frequency offset estimation should be possible.
A function of the long preamble sequence generated considering the foregoing conditions will now be described herein below.
(1) A sequence obtained by repeating a length-64 sequence 4 times is used for time offset estimation and coarse frequency offset estimation.
(2) A sequence obtained by repeating a length-128 sequence 2 times is used for fine frequency offset estimation.
As a result, the long preamble sequence has the following uses in the OFDM communication system.
(1) The long preamble sequence is used as a first preamble sequence of a downlink protocol data unit (PDU).
(2) The long preamble sequence is used for initial ranging.
(3) The long preamble sequence is used for bandwidth request ranging.
Further, the short preamble sequence has the following uses in the OFDM communication system.
(1) The short preamble sequence is used as an uplink data preamble sequence.
(2) The short preamble sequence is used for periodic ranging.
In the OFDM communication system, because accurate synchronization is acquired by performing initial ranging and periodic ranging, the uplink preamble sequence is mainly used for channel estimation. For channel estimation, PAPR, performance, and complexity should be taken into consideration. In the case of the existing short preamble sequence, a PAPR shows 3.5805 [dB], and various channel estimation algorithms such as a minimum mean square error (MMSE) algorithm and a least square (LS) algorithm are used.
FIG. 3 is a diagram illustrating a mapping relation between subcarriers and a preamble sequence during IFFT processing in a conventional OFDM communication system. It is assumed in FIG. 3 that if the number of all of the subcarriers for an OFDM communication system is 256, the 256 subcarriers include −128th to 127th subcarriers, and if the number of subcarriers actually in use is 200, the 200 subcarriers include −100th, . . . , −1st,1st, . . . , 100th subcarriers.
In FIG. 3, input numerals at an IFFT processor's front end represent frequency components, i.e., unique numbers of subcarriers. The reason for inserting null data, or 0-data, into a 0th subcarrier is because the 0th subcarrier, after performing IFFT, represents a reference point of a preamble sequence in a time domain, i.e., represents a DC (Direct Current) component in a time domain. In addition, null data is inserted into 28 subcarriers of the −128th to −101st subcarriers and 27 subcarriers of the 101st to 127th subcarriers, excluding the 200 subcarriers actually in use and the 0th subcarrier. The reason for inserting null data into 28 subcarriers of the −128th to −101st subcarriers and 27 subcarriers of the 101st to 127th subcarriers is to provide a guard interval in a frequency domain because the 28 subcarriers of the −128th to −101st subcarriers and the 27 subcarriers of the 101st to 127th subcarriers correspond to a high frequency band in the frequency domain.
As a result, if a frequency-domain preamble sequence of S(−100:100) or P(−100:100) is applied to an IFFT processor, the IFFT processor maps the frequency-domain preamble sequence of S(−100:100) or P(−100:100) to corresponding subcarriers, IFFT-transforms the mapped preamble sequence, and outputs a time-domain preamble sequence.
FIG. 4 is a block diagram illustrating a structure of a transmitter in a conventional OFDM communication system. Referring to FIG. 4, if information bits to be transmitted are generated, the information bits are applied to a symbol mapper 411. The symbol mapper 411 symbol-maps the input information bits by a preset modulation scheme, and then provides the symbol-mapped bits to a serial-to-parallel (S/P) converter 413. Here, quadrature phase shift keying (QPSK) or 16-ary quadrature amplitude modulation (16QAM) can be used for the modulation scheme. The serial-to-parallel converter 413 parallel-converts symbols received from the symbol mapper 411 so that the number of the received symbols is matched to an A-point, which is the number of inputs of an inverse fast Fourier transformer (IFFT processor) 419, and then provides the parallel-converted symbols to a selector 417. A preamble sequence generator 415, under the control of a controller (not shown), generates a corresponding preamble sequence and provides the generated preamble sequence to the selector 417. The selector 417 selects a signal output from the serial-to-parallel converter 413 or a signal output from the preamble sequence generator 415 according to scheduling at a corresponding time, and provides the selected signal to the IFFT processor 419.
The IFFT processor 419 performs A-point IFFT on a signal output from the S/P converter 413 or the preamble sequence generator 415, and provides its output to a parallel-to-serial (P/S) converter 421. In addition to the signal output from the IFFT processor 419, a cyclic prefix with a length L is applied to the parallel-to-serial converter 421. The parallel-to-serial converter 421 serial-converts the signal output from the IFFT processor 419 and the cyclic prefix, and provides its output to a digital-to-analog (D/A) converter 423. The digital-to-analog converter 423 analog-converts a signal output from the parallel-to-serial converter 421, and provides the analog-converted signal to a radio frequency (RF) processor 425. The RF processor 425 includes a filter, and RF-processes a signal output from the digital-to-analog converter 423 so that it can be transmitted over the air. The RF signal is then transmitted via an antenna.
Now, a description will be made of disadvantages of a preamble sequence generated in a conventional OFDM communication system, and a method for generating the preamble sequence.
(1) In the case of a short preamble sequence used in the existing OFDM communication system, a PAPR is 3.5805 [dB], a relatively large value higher than 3 [dB].
As described above, a preamble sequence improves the OFDM communication system when a PAPR is minimized. Generally, in data transmission, the value of 3.5805 [dB] can be regarded as a relatively small figure that does not affect performance of the OFDM communication system. However, when a preamble sequence used in estimating an initial parameter in the OFDM communication system is taken into consideration, the value of 3.5805 [dB] can become a figure that causes performance deterioration of the OFDM communication system. That is, the preamble sequence must be designed so that its PAPR has a value below 3 [dB] for initial parameter estimation of the OFDM communication system. However, in the case of a short preamble sequence used in the existing OFDM communication system, a PAPR of 3.5805 [dB], a value higher than 3 [dB], causes performance deterioration of the OFDM communication system.
(2) In the existing OFDM communication system, a full search method must be used in order to acquire a preamble sequence having a minimal PAPR. However, the full search method has a long processing time undesirably.
Assuming that in the OFDM communication system, a length of a preamble sequence is X and the number of types of values that elements of the preamble sequence can have is Y, if the full search is performed in order to acquire a preamble sequence having the minimum PAPR, the search must be performed YX times. For example, if Y=2 and X=100, the search must be performed 2100 times in order to acquire a preamble sequence having the minimal PAPR. However, the number of 2100 operations is a very large number, which creates a huge load on the OFDM communication system.
In order to solve such problems, there are demands for a preamble sequence having a minimum PAPR, i.e., a PAPR below 3 [dB] at the maximum, and a method for generating a preamble sequence having the minimum PAPR with the minimized number of operations within the shortest time.